Plasma processing systems are used in semiconductor manufacturing to remove material from or to deposit material on a workpiece (e.g., semiconductor substrate) in the process of making integrated circuit (IC) devices. A key factor in obtaining the highest yield and overall quality of ICs is the uniformity of the plasma etching and deposition processes.
A problem that has plagued prior art plasma processing systems is the control of the plasma to obtain uniform etching and deposition. In plasma processing systems, one factor affecting the degree of etch or deposition uniformity is the spatial uniformity of the plasma density above the workpiece. The latter is dictated by the design of the overall system, and in particular the design of the electrodes (or plasma source) used to create the plasma.
One approach to improving etching and deposition uniformity has been to use a segmented electrode comprising a plurality of metallic electrode segments separated by an insulator. Each electrode segment is electrically connected to an RF power supply that provides power to each electrode segment. Control of the frequency, amplitude and phase of the RF power delivered to each electrode segment can control the plasma density profile of the plasma, and hence etch and deposition characteristics of the system.
Certain types of segmented electrodes have plates of silicon affixed to the side of the electrode segments that face the plasma. These silicon plates are generally many times the thickness of a typical silicon wafer, e.g., about 5-10 mm or so. The silicon plates are used to prevent erosion of the metallic electrode segments to which they are affixed. In the absence of the silicon plates, the high-energy ions in the plasma can etch the electrode and deposit electrode material onto the workpiece, which can affect the performance of devices made from the workpiece. In other words, the electrode can be an unintended sputtering target with the workpiece being deposited with electrode material. The use of protective silicon plates eliminates contamination caused by (high energy) etching the electrode when the workpiece is also made of silicon.
Another advantage of using protective silicon plates on the electrode segments is the etched silicon can act as a source of scavenging specie in the plasma. For example, in some oxide etch applications involving fluorocarbon chemistry, introducing silicon to the plasma above the substrate can lead to the scavenging of fluorine radical. This, in turn, leads to improved etch selectivity between silicon dioxide and silicon. This effect is particularly desirable when etching a silicon dioxide film formed on the workpiece.
The objective of using segmented electrodes in plasma processing is to allow the spatial distribution of the plasma density to be adjusted locally across the workpiece to achieve spatial uniformity of plasma etch and deposition. As mentioned above, each electrode segment is driven by an RF power source whose frequency, amplitude, and phase are controlled separately. Since the field beneath each electrode can change, the amount of material eroded from each silicon plate can differ between electrode segments. For example, upon processing of about 5000 to 10000 wafers (corresponding to about 300 hours RF time), this differential erosion can lead to thickness variations of several millimeters between adjacent silicon plates. Due to the narrow spacing (i.e. ˜20 mm) between the electrode and the wafer, common for high aspect ratio, narrow gap capacitively coupled plasma (CCP), a thickness variation of only a few millimeters corresponds to approximately 5 to 10% of the electrode-workpiece spacing. This variation can dramatically affect process conditions. For example, local workpiece etch rates can change by as much as 500 to 1000 Angstroms/minute.
In a reactor for performing a deposition operation of the chemical vapor deposition (CVD) type, an electrode may be provided at the top of the plasma region to capacitively couple RF power into the plasma. A certain amount of the deposition material generated in the plasma may be deposited on the electrode in a manner that will give the electrode a varying thickness, particularly if the electrode is a segmented electrode and the supply of RF power to each segment is individually controlled. The resulting electrode thickness variation can adversely affect the distribution of RF power within the plasma and hence the spatial uniformity of the plasma.